High-frequency backscatter from terrain with cement-block walls

The measurements of high-frequency (HF) radar cross sections (RCSs)
\sigma
of trees on a large flat field were reported a few years ago by Steele. This work used a special adaptation of the standing-wave method to 26 MHz. Shortly afterward, the present author used the same apparatus to measure the (RCSs) of cement walls erected on the same terrain. Wall sizes ranged from 5 ft high by 10 ft wide to 20 by 20 ft. Angles of elevation
\psi
ranged from 2.5 to
22.5\deg
. Using vertical polarization (
E
field in the plane of incidence), cross sections increased with wall size. When a wall was thoroughly dampened,
\sigma
increased by 3 to 13 dB, depending on
\psi
, and wall size over
\sigma
was obtained when the wall was dry or partly damp. Chicken wire covering increased the 10- by 10-ft wall's cross section by 10 dB. Using horizontal polarization, cross sections were immeasurable for the smaller walls. For the 20- by 20-ft wall,
\sigma
was a smoothly increasing function with
\psi
. When this wall was wet, its cross section increased by 4 dB. The wall-ground combination was treated as a corner reflector, using a theory based on physical optics. The theory compares fairly well to experimental cross sections for the 20- by 20-ft wall. Extrapolation of the theory suggests that a 150-ft high by 100-ft wide building could have a cross section approaching
10^{4}
m
²
at 26 MHz.